Many patients at risk of cardiac ischemia have pacemakers, implantable cardioverter defibrillators (ICDs), or other medical devices implanted therein. Electrocardiograms (ECG) are useful for diagnosing ischemia and locating damaged areas within the heart. Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. ECGs are composed of various waves and segments that represent the heart depolarizing and repolarizing. The ST segment represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization. While P-waves, R-waves and T-waves may be generally considered features of a surface ECGs, for convenience and generality, herein the terms R-wave, T-wave and P-wave are also used to refer to the corresponding internal cardiac signal, such as an intra-cardiac electrogram (IEGM).
Techniques have been developed for detecting cardiac ischemia using implanted medical devices. Some conventional IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the ST segment from the baseline of the IEGM that occur during cardiac ischemia. Elevation of the ST segment in an IEGM may result when there are abnormalities in the polarizations of cardiac tissue during an acute myocardial infraction (AMI). Deviation of the ST segment from a baseline is a result of injury to cardiac muscle, changes in the synchronization of ventricular muscle depolarization, drug or electrolyte influences, or the like. However, not all ST segment shifts are indicative of AMI or other injury to the cardiac muscle. Instead, a ST segment shift above or below the baseline may result because of “axis shifts”, electrical noise, cardiac pacing, high sinus or tachycardia cardiac rates that distort the IEGM waveform.
Heretofore, conventional pacemakers or defibrillators have experienced some difficulties in identifying ST segment shifts that are not indicative of AMI, ischemia or other injury to the myocardial muscle. One type of ischemia is “demand ischemia” which becomes apparent when the oxygen demand increases sufficiently. Demand ischemia will often lead to anginal pain, such as during physical exertion. The anginal pain is often associated with a predictable threshold of physical activity. Hence, a patient may experience chest pains (e.g., angina) while exercising. Conventional IMDs do not monitor any relation between patient activity and ischemic condition, nor provide any indication of how much activity a patient endures before suffering anginal pain.
A need exists for methods and systems to track a patient's level of activity before the onset of anginal pain and/or ischemia. Additionally, a need remains for methods and systems determining the level of activity a patient may endure over a period of time without suffering from anginal pain.